Dynamic Point Selection (DPS) is a key downlink Coordinated Multipoint (CoMP) technique that switches the serving data Transmission Point (TP) of a User Equipment (UE) dynamically among the UE's cooperating sets of TPs without requiring a cell handover. A TP is defined as a set of geographically collocated transmit antennas. The prominent benefits of Dynamic Point Selection include performance improvement due to TP selection-diversity gains and dynamic UE load balancing benefits.
DPS switches the serving TP of the UE based on the UE's channel and the cell load conditions. This TP switching can be done on a very fast time scale without requiring an elaborate handover procedure.
In this regard, it is described in document [1], that LTE Release 11 allows cell-agnostic DL CoMP operation by introducing a new framework for multi-cell channel state information (CSI) feedback from the UE based on an enhanced downlink reference signal structure. Each UE is configured a CoMP measurement set, which is a set of CSI reference signal (CSI-RS) resources for which the UE is required to measure and feedback the CSI. The configuration of this CoMP measurement set is UE-specific and can be determined based on the UE's mobility measurements (for example, RSRP) or the UE's uplink sounding reference signal (SRS) transmissions. Typically, each CSI-RS resource would correspond to a TP.
Moreover, it is mentioned that for each TP included in the UE's CoMP measurement set, the UE is configured with at least one independent CSI process feedback. The UE determines the CSI of a configured CSI process by using the associated non zero power (NZP) CSI-RS resources (which are used to measure the desired signal power) and the newly defined interference measurement resources (IMRs) (which are used to measure the interference power). Up to a maximum of three different NZP CSI-RS and IMRs can be configured for a UE, which limits the CoMP measurement set to three TPs for each UE.
The CoMP transmission set of a UE (which is the set of cooperating TPs from within which a UE's serving TP can be selected) comprises the top N TPs with the largest Reference-Signal Received Power (RSRP) values measured by the UE from within the “liquid cluster” [1] of the UE's serving TP.
The cell on which UE performs RRC (radio resource control) attach procedure, is called primary TP and the other cells in the CoMP transmission set are called serving TPs if UE switches to those cells.
Conventional DPS schemes, as described for example in document [1], consider the spectral efficiency and/or cell load to determine the current serving TP for a UE.
However, the conventional DPS schemes do not take into account the burstiness of the UE traffic and the resulting dynamics in the inter-cell interference caused by switching a UE to a different TP. For example, in case of load based DPS, a UE may be switched to a lightly loaded TP, even though this new TP is not the best TP for the UE in terms of spectral efficiency. This can increase the resource usage (in time and/or frequency) of the UE and hence, increase the inter-cell interference caused by data transmissions to the UEs. This effect is more pronounced in case of realistic unbalanced bursty traffic conditions as shown by the simulation results. The main problem lies in the fact that these current DPS schemes in [1] and [2] are greedy user schemes that are optimized for maximizing a single UE's throughput without considering its impact on the overall system performance. In [2], they have also considered multi-user diversity (MUD) gain factor in the UE switching metric (called in this application as MUD-IL-DPS when this MUD gain factor is applied to IL-DPS). The MUD-IL-DPS scheme accounts for the multi-user diversity gain while determining the throughput estimates of the UE in different candidate TPs. This MUD gain is obtained due to the opportunistic scheduling of the UEs by the PF scheduler. In document [2], an online estimation scheme of this MUD gain is described that provides better accuracy.
However, this method may not be practical as it requires the past knowledge of scheduling decisions and the scheduled data rates of all the active UEs in a cell over a fixed time window. For simplicity sake, in our simulation evaluation comparison, we use instead a simple mathematical formula for MUD gains as given in document [3] in the TP switching metric which only depends on the total number of active UEs competing for the resources.
The Geometric Mean (GM) of the UE throughputs is used as the metric to evaluate the system performance of the considered DPS schemes. This GM metric is shown to be a single metric that effectively captures the overall system performance, in case of the commonly used Proportional-Fairness (PF) based scheduling strategy. The typical industry-practice of using two metrics, average UE throughput, and cell-edge UE throughput can yield conflicting conclusions. Besides, it is noted that the proportional fairness metric is designed to maximize the geometric mean of UE throughputs, and hence using a different performance metric than what the scheduling strategy is maximizing may cause system designers to draw wrong conclusions.